Heating arrangement utilizing microwaves



N. B. AGDUR ET AL BEAT ING ARRANGEMENT UT ILI Z ING MICROWAVES' Nov. 11,1969 Filed ob". e, we?

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HEATING ARRANGEMENT UTILIZING MICROWAYES Filed Oct. 6, 1967 2Sheets-Sheet 2 1 v l v w ,1 v I u \j g a 1 TJTENTORQ Nu- BG\TH.. RGDQI.PIC. 01.0! umnaQ cram.

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3,478,187 HEATING ARRANGEMENT UTILIZING MICROWAVES Nils Bertil Agdur,Danderyd, and Per Olof Gunnar Hedvall, Akersberga, Sweden, assignors toSkandinaviska Processinstrument AB, Stockholm, Sweden, a company ofSweden Filed Oct. 6, 1967, Ser. No. 673,506 Claims priority, applicationSweden, Oct. 19, 1966, 14,242/66 Int. Cl. Hb 9/06 US. Cl. 21910.55 12Claims ABSTRACT OF THE DISCLOSURE There is disclosed a microwave heatingapparatus which includes a source of microwave power which generatesmicrowaves having a given operating wavelength. A microwave structure isconnected to the source of microwave power. The structure includes aconductive base plate from which extend a plurality of T-shapedcrosssection microwave elements disposed in regular array. The elementsare spaced from each other by a distance which is so much smaller thanthe free-space operating wavelength of the microwaves that themicrowaves propagate along the structure with 'a phase velocity which isless than the free-space velocity of the microwaves.

The present invention refers to apparatus for heating materials by meansof microwave energy from a microwave source.

An object of the invention is to provide e.g. the paper and woodindustry with a simple and reliable device for heating and for drying ofpaper, pulp and other cellulose products, wood, glue, dyes etc. In thisdevice the heating (drying) power can be controlled simply andinstantaneously by temperature and humidity sensors. Another object ofthe invention is to provide a simple device for heating, boiling andfrying foodstuffs. In such a device the preparation times will be muchshorter than for conventional methods.

Apparatus according to this invention is characterized in that amicrowave source is coupled to a microwave structure having the propertythat the microwave energy propagates along the structure while remainingconcentrated to a narrow region near the structure and in that thematerial to be heated is placed in or passes this narrow region.

The invention will be described in detail in connection with theaccompanying schematic drawings, where FIG. 1 shows a heating devicewith central microwave I power input,

FIG. 2 shows a part of the device of FIG. I seen from above and withsome field lines indicated,

FIG. 3 shows a part of the device of BIG. 1 with indicated field lines,

FIG. 4 shows a modified cross section of the elements for a deviceaccording to FIG. 1,

FIG. 5 shows the vertical distribution of energy between two elements ofthe device,

FIG. 6 shows a modification of the arrangement of FIG. .2,

FIG. 7 shows heating device with circumferential power input, and

FIG. 8 shows a modification of the device of FIG. 1 such that the poweris fed in at one end of the structure.

The device shown in FIG. 1 comprises amicrowave input 10-11 and amicrowave structure (heating table) 13-14-15. The input consists of awaveguide 10 and a coupling loop 11 for the microwave power. The heating3,478,187 Patented Nov. 11, 1969 ICC table consists of a number ofmutually similar T-shaped metal elements 14 mounted on a metal baseplate 13. The coupling loop 11 has one end directly connected to one ofthe elements 14, the other end projects into the central part of thewaveguide 10. Thus the microwave power from the waveguide 10 is fed intoand propagated both ways by the microwave structure formed by theelements 14 and the base plate 13. At either end of this structure therehave been placed essentially vertical metal plates 15 furnished withslit-shaped quarter-wavelength chokes 16. The purpose of the metalplates 15 is to reflect back towards the center of the structure thatpart of the microwave power that has not been absorbed in the materialto be heated. The material is passed through the slits 16 andimmediately above the elements 14. To protect the microwave sourceagainst excessive reflected powerfor instance when no material to beheated is present in the device-energy absorbers 17 have been placedadjacent to the metal plates 15. The energy absorbers 17 can compriseglass tubes with water running through them in which part of the excessmicrowave power will be absorbed and converted to heat.

From FIG. 2 it is seen that the elements 14 are elongated and extendperpendicular to the direction of transmission of the microwave poweralong the microwave structure (heating table) 13 14-15. Further, theelements are arranged in such a way that the distance betweencorresponding points on adjacent elements is less than half thefree-space operating wavelength of the microwaves. This distance caneither be constant as shown in FIG. 1 or can increase gradually awayfrom the input (increasing phase velocity) or can decrease gradually(decreasing phase velocity).

A microwave structure such as the one shown in FIGS. 1-3 has theproperty that the microwaves propagate with a phase velocity that isless than the free-space velocity. The microwave energy will then beclosely bound to the microwave structure. The field configuration isoutlined in FIGS. 2P3. The electric field (solid lines) is mainlyconcentrated near the gaps between the flanges of adjacent elements,whereas the magnetic field lines (broken lines) mainly close round thewebs of the elements 14. The concentration of the electric field-withconsequent high energy densityclose to the upper surface of themicrowave structure can be made more pronounced by means of elementshaving sharp edges as shown in FIG. 4.

FIG. 5 shows schematically how the electric microwave energy isdistributed along a vertical line between two elements of the type shownon the left in the figure. It is seen that the energy is concentratednear. the level of the flanges and decreases exponentially above thestructure.

The material to be heated (dried) is placed on top of the structure(heating table) where it'will be exposed to strong electric fields.Dissipation in the material will causepart of the microwave power to beconverted into heat inside the material and the propagating wave will bedamped. Because the field concentrates near to the gaps between theelements 14 there exists a risk of local burns in solid materials. Thiscan be avoided if such materials, e.g. paper, are made to move along thestructure. If the dielectric constant of the material is greater thanthat of air, the electric energy will tend to concentrate in thematerial. The exponential decrease of the energy density will also beless marked and the microwave power can permeate and heat even fairlythick materials.

FIG. 6 shows a modified microwave sructure. Instead of elongatedelements 14 as in FIG. 2 the elements 64 are essentially rectangularwhen seen from above. The elements 64 comprise a vertical stern and ahorizontal flange so that a vertical section through an element willstill be similar to that shown in FIG. 5. The elements 64 are arrangedin a regular two-dimensional array on a base plate 13in such'a way thatthe distance between corresponding points on two diagonally adjacentelements is less than half the free-space operating wavelength of themicrowaves. The elements then form a plane microwave structure thatallows the microwaves to propagate in several directions with a phasevelocity less than the free-space velocity. The field configuration issuch that the electric field is concentrated to the region between theedges of the flanges of the T-shaped elements 64 and the magnetic fieldlines mainly close around the stems of these elements.

The embodiment shown in FIG. 7 comprises a number of concentric circularelements 74 with T-shaped cross section. The elements are mounted on acircular base plate 13. A vertical reflector tube 75 is connected to therim of the base plate. The microwave power is fed in from a circularlycylindrical waveguide 70. The coupling loops 71 have one end connectedto the inner wall of the tube 75, the other end projects into thewave-guide 70. The advantage of this circumferential power input is thatthe attenuation of the microwaves due to energyabsorption in thematerial will be counteracted by the geometric focussing of the waves.

In the embodiment shown in FIG. 8 one part 82 of the microwave structure(heating table) has a comparatively small curvature and the other part83 has a greater curvature. The microwave input 80-81 is connected to areflector at the free end of the second part 83. The elements 84 of themicrowave structure 82-83 are T-shaped as in the abovedescribedembodiments. The material to be heated (dried), eg a paper web 89,touches the slightly curved part 82 of the microwave structure.

Many'other possible arrangements come within the scope of the inventionas defined in the appended claims. Thus the microwave structure canbemade cylindrical and arranged for internal or external heating oftubes. In general the shape of the structure may be adapted to suit thematerial for which it is to be used.

Further, the T-shaped elements and base plate need not be madeseparately but may, for instance, be manufactured by bending(corrugating) sheet-metal, e.g. copper, to obtain the desired profile.In this way microwave structure will 'be made up of a continuous coppersurface without soldering joints which will decrease the undesirablelosses of microwave power. At the same time the manufacturing procedureshould be both simple and inexpensive.

The microwave input can also be modified in several ways. Thus themicrowave structure may form an integral part of a microwave generator,for instance a distributed transistor amplifier.

What is claimed is:

1. Heating apparatus utilizing microwave power comprising a source ofmicrowave power wherein the microwaves have a given operatingwavelength, and a microwave structure connected to said source ofmicrowave power, said microwave structure including a plurality ofmutually similar microwave elementspaced from each other in a reglararray, the distance between corresponding points of adjust microwaveelements being so much smaller than half the free-space operatingwavelength of the microwaves and said microwave elements cooperatingwith each other so that the microwaves propagate along the microwavestructure with a phase velocity less than the free-space velocitywhereby microwave energy is concentrated in the microwave structureitself and in a region adjacent to the microwave structure.

2. Apparatus according to claim 1 wherein said microwave structurecomprises a conductive base plate and the microwave elements areelongate elements made of conductive material and having a generallyT-shaped crosssection, the foot of each element being in immediateelectric contact with said conductive base plate.

3. Apparatus according to claim 2 wherein the generally T-shapedcross-section of the microwave elements has a gradual transition'between its cross arm and its long arm and that the cross arm has asharp edge.

4. Apparatus acording to claim 2 wherein the T-shaped elements and thebase plate are manufactured integrally by bending sheet-metal to obtainthe desired profile.

5. Apparatus according to claim 2 wherein the microwave elements areshaped as concentic circles.

6. Apparatus according to claim 5 wherein the microwave source isconnected to the circumference of the circular structure.

7. Apparatus according to claim 1, wherein said microwave structurecomprises a conductive base plate, and said microwave elements arearranged in a two-dimensional array, said elements being made ofconductive material and of a substantially T-shaped cross-section havinga top flange and a stem, the stems of the elements being in immediateelectric contact with the conductive base plate.

8. Apparatus according to claim 1 wherein the microwave structure isbent and comprises at least one part with small curvature and a secondpart with more pro nounced curvature, said microwave source beingconnected to the second part.

9. Apparatus according to claim 1 wherein the microwave source isconnected to a central part of the microwave structure.

10. Apparatus according to claim 1 wherein the microwave structureincludes, along at least one edge, a conductive surface member disposedperpendicular to the direction of propagation of the microwaves forreflecting microwave power toward the material to be heated.

11. Apparatus accordin gto claim 10 wherein the conductive surfacemember includes an opening for passage of the material to be heated.

12. Apparatus according to claim 1 wherein the microwave structureincludes microwave energy absorber means for absorbing such microwavepower as has not been absorbed in the material to be heated.

References Cited UNITED STATES PATENTS 2,492,187 12/1949 Rusca 219-10612,960,777 11/1960 Doll 219-10.61 X 3,205,334 9/1965 'Manwaring2l9--10.61 3,263,052 7/1966 Jeppson et a1. 21910.61 X 3,271,552 9/1966Krajewski 2l9-10..55

JOSEPH V. TRUHE, Primary Examiner L. H. Bender, Assistant Examiner

